The present invention relates generally to satellite navigation and more specifically to satellite acquisition and tracking systems.
Modem Global Positioning System (GPS) receivers typically include a real time clock that is used to estimate visible satellites upon acquisition. The GPS system by nature provides time information to the user after satellite acquisition occurs. When a GPS system device is powered down, a battery backed real time clock maintains a time reference such that the position of satellites may be readily determined when the GPS system device is subsequently powered on. This determination is based upon previously stored position data of the system and almanac data of the satellites along with the time reference provided by the real time clock. However, an accurate real time clock provides increased expense to a GPS system device. As the cost of consumer grade GPS receivers continues to fall, methods and techniques to reduce the cost are becoming increasingly important to compete in the marketplace. As a result, it is desirable to be able to eliminate any need for a real time clock in a GPS system device and at the same time minimize impact on satellite acquisition times.
The present invention eliminates the need for a real time clock as a component of a GPS system device or the like. The need for a real time clock is obviated by utilizing information related to satellite positions over predetermined time intervals spanning the orbital period of the space vehicles. When the GPS system device is powered on, the GPS system device assumes that the position of the system is nearby the last computed and stored position when power was last removed. The time uncertainty is solved using knowledge of the satellite constellation over the orbital period. The GPS system device pre-computes the highest satellite visible for each predetermined interval, for example a period of 2 hours, during the course of an orbital period at the present position of the GPS system device. In the case of the GPS Navstar system, completion of an approximate 12 hour orbital period at 2-hour intervals results in a list of 6 satellites. When the receiver is powered on, each channel of a parallel channel receiver is assigned one of the satellites from the pre-computed list of satellites. When one of the satellites is acquired, time uncertainty is eliminated or reduced, in part by knowing which satellite is acquired versus its relationship to the list. Once a satellite signal is acquired, the handover word in the navigation data stream allows the receiver to recompute an actual list of visible satellites after which normal acquisition can commence. Satellites that are not visible are removed from the acquisition list and replaced by those that are visible.